How low does it go? Too few Galactic satellites with standard reionization quenching

Abstract

A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than V peak 20 km s −1 , rendering the majority of haloes below this scale completely dark. We use a suite of cosmological zoom simulations of Milky Way-like haloes that include central Milky Way disk galaxy potentials to investigate the relationship between subhaloes and ultrafaint galaxies. We find that there are far too few subhaloes within 50 kpc of the Milky Way that had V peak 20 km s −1 to account for the number of ultrafaint galaxies already known within that volume today. In order to match the observed count, we must populate subhaloes down to V peak 6 km s −1 with ultrafaint dwarfs. The required haloes have peak virial temperatures as low as 1, 500 K, well below the atomic hydrogen cooling limit of 10 4 K. Allowing for the possibility that the Large Magellanic Cloud contributes several of the satellites within 50 kpc could potentially raise this threshold to 10 km s −1 (4, 000 K), still below the atomic cooling limit and far below the nominal reionization threshold.

Milky Way Satellite Census. II. Galaxy–Halo Connection Constraints Including the Impact of the Large Magellanic Cloud

“…7 These results improve upon the minimum halo mass constraint derived from classical and SDSS satellites (Nadler et al 2019b) by a factor of 2.5, and they are consistent with the constraints reported in Jethwa et al (2018). Moreover, these upper limits are not in significant tension with the expected atomic cooling limit of V peak ≈ 20 km s −1 , contrary to recent studies based on the radial MW satellite distribution (e.g., Graus et al 2019), and consistent with the findings in Bose et al (2019). We caution that the median galaxy occupation fraction shown in Figure 6 is driven by the assumed functional form in Equation 3, and is therefore arbitrary.…”

The little things matter: relating the abundance of ultrafaint satellites to the hosts’ assembly history

“…The main conclusion drawn from Fig. 7 is at odds with the results of Graus et al (2019) who, after including the effects of disc destruction and assuming the standard model of reionisation, found about 10 times fewer satellites within 50 kpc of the halo centre than the observed number in this region of the Milky Way. The reason why predicts far more ultrafaint satellites in the inner regions of galactic haloes is simply the inclusion of 'orphan' galaxies, that is galaxies whose dark matter haloes have been lost due to numerical resolution effects as the number of particles in the subhalo drops below a certain level (20 particles in the case of subhaloes).…”

“…The radial profiles that we find in this case are consistent with those of Graus et al (2019). Including the orphans obviates the need to populate extremely small subhaloes, as Graus et al (2019) had to do in their simulations to obtain as many ultrafaint satellites as observed. In fact, in the smallest haloes that ever form a galaxy have peak ∼ 16.8 kms −1 .…”

“…Zentner et al 2005;D'Onghia et al 2010;Zhu et al 2016;Sawala et al 2017;Garrison-Kimmel et al 2017;Richings et al 2018;Simpson et al 2018;Buck et al 2019). Graus et al (2019) have suggested that when disc disruption is accounted for, ΛCDM simulations are inconsistent with the abundance of satellites observed in the inner parts of the Milky Way. They find that under the assumption of "standard reionisation quenching" -in which galaxies with peak circular velocity peak ≤ 20 kms −1 no longer accrete gas after reionisation (Okamoto et al 2008(Okamoto et al , 2010 there are far fewer subhaloes within 50 kpc of the centre in the dark matter-only simulations than the observed number of satellites in the Milky Way.…”

“…The reason why predicts far more ultrafaint satellites in the inner regions of galactic haloes is simply the inclusion of 'orphan' galaxies, that is galaxies whose dark matter haloes have been lost due to numerical resolution effects as the number of particles in the subhalo drops below a certain level (20 particles in the case of subhaloes). The resolution of the dark matter-only simulations used by Graus et al (2019) is 3 × 10 4 M . This is about a factor of 3 lower resolution than the 'Level-2' Aquarius simulations of galactic haloes (Springel et al 2008) for which Newton et al (2018a) have shown orphan galaxies are important, particularly in the central regions.…”